It would seem like this variable also cannot be read with variable_ functions, and not just that, but it’s also falsely flagged as an instance variable in fnames file, which caused my existing workaround generator to not make a function for it

object_index*@
id*@
solid@
persistent@
mask_index@
instance_count*@
instance_id*@
instance_find(obj,n)
instance_exists(obj)

A workaround would be to do

gml_var_add("instance_count*", function(l_set, l_val) {
		return instance_count;
});

on game start.